INTRODUCTION
1. A volumetric reaction is defined through macros in the source code, which is preferential over a surface reaction to minimize the numerical instability caused by large intra-cell gradients. As water vapor is a small fraction of the overall flue gas and the fraction that condenses3. Proportional to the water vapor mass condensing, there is a latent heat release for that cell.is even smaller, gradients from cell face to cell face will be very large, causing solver oscillations.A major market barrier to upgrading the efficiency of or fuel-switching with a heating system is the cost associated with the chimney liner and its installation. According to one utility's cost breakdown, the chimney liner constitutes 25% of the total conversion cost of fuel-switching. This conversion is subject to provisions of both NFPA 54 National Fuel Gas Code (NFGC) and NFPA 211 Standard for Chimneys, Fireplaces, Vents and Solid Fuel Burning Appliances. This is, however, in contrast to the costs brought upon by pitting corrosion, vent failure, and potential for unsafe venting of products of combustion (DeWerth, 1983). With respect to pertinent codes, it is the responsibility of both the regulators and regulated community to periodically re-examine the underlying assumptions and technical process behind standing code guidance. The research community may aid in the process, as with any unresolved technical issues that arise in that applicable code and installation practice evaluation can be addressed by modeling and possibly laboratory investigation to justify those code compliance interpretations.MODEL DEVELOPMENT AND ASSUMPTIONSOur showroom opening times are Monday to Saturday, 10am to 5.30pm, Sunday 11am to 4pmA number of showroom special offers are always available. Call in and see us -you won''t be disappointed.The objective in using CFD modeling is to use a sophisticated tool selected both to compare to VENT-II simulation results and experimental data, while exploring the venting dynamics of gas-fired boilers versus relative to that of fan-assisted furnaces, the subject of prior study and the venting tables. VENT-II is a one dimensional nodal model, solving a reduced form of the Navier-Stokes equations and a semi-empirical condensation model at the interior flue surface. In addition to cycling and ambient conditions, boundary conditions at the appliance outlets characterize flue gas properties relative to user-defined cycling, temperature profiles, firing rates, and percentage of excess air. FLUENT version 6.3 is the CFD simulation software used, with turbulence modeled using the k-[epsilon] model. The fluid domain includes the chimney interior and the ambient surroundings, primarily included for solution stability, and the solid domain is the chimney itself for heat transfer modeling. Sensitivity of condensation results to meshing of the chimney interior was explored, which is shown in Figure 1 in addition to this computational domain. Each simulation and chimney configuration utilized this mesh.In the current study, the project team conducted a technical evaluation and laboratory investigation of factors affecting venting requirements for gas vent systems in a representative utility service territory fuel-switching, thus vent system upgrades, are more common, the U.S. Northeast. Focusing on the potential for relining requirements during these upgrades, this study considers venting systems with masonry chimneys. Hydronic (hot water) boilers are prevalent in this region, which were treated as equivalent to furnaces in the original development of the NFGC (Philips, 1994):We are able to offer a full fitting service, carried out by fully qualified Gas Safe and H.E.T.A.S. engineers.Whether you simply want to change the trim on your fire or are looking to create a truly bespoke, individual fireplace we are here to help and guide you.Initial screening CFD results that do not directly model condensation show order of magnitude agreements with the VENT-II analysis, there are known gaps in modeling physics concerning the mass and heat transfer associated with condensation. The source code of FLUENT is not accessible for modification; however it offers many so-called 'access points' where users may program additional physical models to be included with those built-in. This is how condensation is directly modeled, with customizable physical models. To directly model surface condensation in CFD, a so-called User Defined Function (UDF) is programmed which models condensation as a volumetric reaction rate coupled with the latent energy source/sink. To prevent numerical instabilities and solution divergence in the implementation of a UDF during CFD simulation, a simplified approach is taken to the UDF formulation. In brief, the UDF is defined as follows:At the Fireplace Centre we understand how important it is to create a stylish and relaxing living environment.Higher efficient gas-fired appliances in Category I venting systems are more likely to result in condensation within exterior masonry chimneys. The use of vent dampers with hot water boilers reduces the off-cycle flow significantly, reducing the ability of the vent system to dry out between on-cycles. Additionally, boilers with vent dampers have different off-cycle flow characteristics than the furnaces used to develop the Category I appliance venting tables in the NFGC (hereafter referred to as "venting tables").Following initial confirmation that CFD modeling techniques to be used showed good agreement with the software used in the development of the venting tables, VENT-II, a parametric matrix for CFD modeling, was developed and refined in cooperation with the primary UTD sponsor. The metric used to determine agreement is the presence of net surface condensation on the chimney interior, continuously "wet" or "dry" conditions. The parametric modeling matrix incorporates typical physical and operating conditions seen in areas where oil-fired appliances are prevalent, namely the Northeastern U.S.You can find us at Northonthorpe Mills, Wakefield Road, Scissett. Telephone 01484 862224 THE FIREPLACE & STOVE CENTRE -WARMING UP YOUR LIFEIf it''s a fireplace you''re looking for, we have everything from small and sweet to large and grand in a wide range of materials, including limestone, marble, granite, stone and wood surrounds in styles to suit everyone.We have a wide range of fires available in both gas and electric and stoves in multi-fuel, wood burning, gas and electric.With this in mind we have put together a range of fires, fireplaces and stoves to cater for all tastes and budgets.
Following initial confirmation that CFD modeling techniques to be used showed good agreement with the software used in the development of the venting tables, VENT-II, a parametric matrix for CFD modeling, was developed and refined in cooperation with the primary UTD sponsor. The metric used to determine agreement is the presence of net surface condensation on the chimney interior, continuously "wet" or "dry" conditions. The parametric modeling matrix incorporates typical physical and operating conditions seen in areas where oil-fired appliances are prevalent, namely the Northeastern U.S.
Author: Paul Glanville, Larry Brand, Shawn Scott
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